Dinoflagellates of the Alexandrium ostenfeldii complex (A. ostenfeldii, A. peruvianum) are capable of
producing different types of neurotoxins: paralytic shellfish toxins (PSTs), spirolides and gymnodimines,
depending on the strain and its geographic origin. While Atlantic and Mediterranean strains have been
reported to produce spirolides, strains originating from the brackish Baltic Sea produce PSTs. Some North
Sea, USA and New Zealand strains contain both toxins. Causes for such intraspecific variability in toxin
production are unknown. We investigated whether salinity affects toxin production and growth rate of 5
A. ostenfeldii/peruvianum strains with brackish water (Baltic Sea) or oceanic (NE Atlantic) origin. The
strains were grown until stationary phase at 7 salinities (6–35), and their growth and toxin production
was monitored. Presence of saxitoxin (STX) genes (sxtA1 and sxtA4 motifs) in each strain was also
analyzed. Salinity significantly affected both growth rate and toxicity of the individual strains but did not
change their major toxin profile. The two Baltic Sea strains exhibited growth at salinities 6–25 and
consistently produced gonyautoxin (GTX) 2, GTX3 and STX. The two North Sea strains grew at salinities
20–35 and produced mainly 20-methyl spirolide G (20mG), whereas the strain originating from the
northern coast of Ireland was able to grow at salinities 15–35, only producing 13-desmethyl spirolide C
(13dmC). The effects of salinity on total cellular toxin concentration and distribution of toxin analogs
were strain-specific. Both saxitoxin gene motifs were present in the Baltic Sea strains, whereas the 2
North Sea strains lacked sxtA4, and the Irish strain lacked both motifs. Thus sxtA4 only seems to be
specific for PST producing strains. The results show that toxin profiles of A. ostenfeldii/peruvianum strains
are predetermined and the production of either spirolides or PSTs cannot be induced by salinity changes.
However, changes in salinity may lead to changed growth rates, total cellular toxin concentrations as
well as relative distribution of the different PST and spirolide analogs, thus affecting the actual toxicity of A. ostenfeldii/peruvianum populations.